Optical gate device, manufacturing method for the device, and system including the device

ABSTRACT

Disclosed herein is a device including first and second optical couplers and a loop optical path. The first optical coupler includes first and second optical paths directionally coupled to each other. The loop optical path includes an optical fiber as a nonlinear optical medium, and connects the first and second optical paths. The second optical coupler includes a third optical path directionally coupled to the loop optical path. The optical fiber has an enough large nonlinear coefficient. The wording of “enough large” means that the nonlinear coefficient is large enough to reduce the length of the optical fiber to such an extent that the optical fiber has a polarization maintaining ability. By using such an optical fiber having an enough large nonlinear coefficient, a relatively short optical fiber can be used as the nonlinear optical medium. Accordingly, it is possible to provide an optical gate device which can suppress a signal rate limit due to chromatic dispersion and can easily cope with polarization dependence of an input optical signal and polarization fluctuations in the loop optical path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical gate device, a manufacturingmethod for the device, and a system including the device.

2. Description of the Related Art

A Mach-Zehnder interferometer (MZI) type optical gate is known as aconventional optical gate device. This optical gate is configured byintegrating a Mach-Zehnder interferometer including first and secondnonlinear optical media each for providing a phase shift on an opticalwaveguide substrate, for example. Probe light as continuous wave (CW)light is equally divided into two components, which are in turn suppliedto the first and second nonlinear optical media. The optical path lengthof the interferometer is set so that output light is not obtained byinterference of the two components of the probe light.

An optical signal is further supplied to one of the first and secondnonlinear optical media. By properly setting the powers of the opticalsignal and the probe light, a converted optical signal synchronous withthe optical signal is switched out from the optical gate. The convertedoptical signal has the same wavelength as that of the probe light.

It has been proposed to use a semiconductor optical amplifier (SOA) aseach of the first and second nonlinear optical media. For example, anInGaAs-SOA having two (both) end faces treated with antireflectioncoatings is used as each nonlinear optical medium in a 1.5 μm band, andthese nonlinear optical media are integrated on an InP/GaInAsP substrateto fabricate an optical gate.

A nonlinear optical loop mirror (NOLM) is known as another conventionaloptical gate device. The NOLM includes a first optical coupler includingfirst and second optical paths directionally coupled to each other, aloop optical path for connecting the first and second optical paths, anda second optical coupler including a third optical path directionallycoupled to the loop optical path.

By forming a part or the whole of the loop optical path from a nonlinearoptical medium and supplying probe light and an optical signalrespectively to the first optical path and the third optical path, aconverted optical signal is output from the second optical path.

An optical fiber is generally used as the nonlinear optical medium inthe NOLM. In particular, a NOLM using a SOA as the nonlinear opticalmedium is referred to as an SLALOM (Semiconductor Laser Amplifier in aLoop Mirror).

The MZI type optical gate is excellent in size reduction andintegration, but its manufacturing technique has not yet beenestablished.

The optical gate having an SOA as the nonlinear optical medium has aproblem that amplified spontaneous emission (ASE) noise added by the SOAhas an adverse effect on fundamental characteristics including asignal-to-noise ratio (SNR).

On the other hand, the NOLM requires a long fiber to obtain a requirednonlinear optical effect. Accordingly, there arises a signal rate limitdue to chromatic dispersion, and it is difficult to cope withpolarization dependence of an input optical signal and polarizationfluctuations in the loop optical path.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalgate device allowing the use of a relatively short optical fiber as thenonlinear optical medium, and to also provide a manufacturing method forthe device and a system including the device.

In accordance with an aspect of the present invention, there is provideda device including first and second optical couplers and a loop opticalpath. The first optical coupler includes first and second optical pathsdirectionally coupled to each other. The loop optical path includes anoptical fiber as a nonlinear optical medium, and connects the first andsecond optical paths. The second optical coupler includes a thirdoptical path directionally coupled to the loop optical path. The opticalfiber has an enough large nonlinear coefficient. The wording of “enoughlarge” means that the nonlinear coefficient is large enough to reducethe length of the optical fiber to such an extent that the optical fiberhas a polarization maintaining ability.

In the present invention, by using such an optical fiber having anenough large nonlinear coefficient, a relatively short optical fiber canbe used as the nonlinear optical medium. Accordingly, it is possible toprovide an optical gate device which can suppress a signal rate limitdue to chromatic dispersion and can easily cope with polarizationdependence of an input optical signal and polarization fluctuations inthe loop optical path.

In accordance with another aspect of the present invention, there areprovided first to third manufacturing methods for a device having afirst optical coupler including first and second optical pathsdirectionally coupled to each other, a nonlinear optical medium forforming a loop optical path connecting the first and second opticalpaths, and a second optical coupler including a third optical pathdirectionally coupled to the loop optical path.

The first manufacturing method comprises the steps of (a) cutting anoptical fiber into a plurality of sections, and (b) arranging theplurality of sections and joining them together so that a conversionband by a third-order nonlinear effect using the nonlinear opticalmedium becomes a maximum, thereby obtaining the nonlinear opticalmedium.

The second manufacturing method comprises the steps of (a) cutting anoptical fiber into a plurality of sections, (b) measuring thedispersions of the plurality of sections, and (c) selecting any of thesections having dispersions small enough to obtain a required conversionband by a third-order nonlinear effect using the nonlinear opticalmedium and joining the selected sections, thereby obtaining thenonlinear optical medium.

The third manufacturing method comprises the steps of (a) measuring adeviation in zero-dispersion wavelength of an optical fiber, (b) cuttingthe optical fiber into a plurality of sections when the deviationexceeds a predetermined range, and making the deviation inzero-dispersion wavelength of each of the sections fall within thepredetermined range, and (c) selecting the optical fiber or the sectionshaving substantially the same zero-dispersion wavelength and joining theselected sections, thereby obtaining the nonlinear optical medium.

In accordance with a further aspect of the present invention, there isprovided a system comprising an optical gate device, a probe lightsource, and first and second optical fiber transmission lines. Theoptical gate device comprises a first optical coupler including firstand second optical paths directionally coupled to each other, anonlinear optical medium for forming a loop optical path connecting thefirst and second optical paths, and a second optical coupler including athird optical path directionally coupled to the loop optical path. Theprobe light source is connected to the first optical path to supplyprobe light to the first optical path. The first optical fibertransmission line is connected to the third optical path to supply anoptical signal to the third optical path. The second optical fibertransmission line is connected to the second optical path to transmit aconverted optical signal output from the second optical path.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a configuration of an MZI type optical gatein the prior art;

FIG. 2 is a graph showing an input-output characteristic of the opticalgate shown in FIG. 1;

FIG. 3 is a diagram showing a configuration of an NOLM in the prior art;

FIG. 4 is a diagram showing a first preferred embodiment of the NOLMaccording to the present invention;

FIG. 5 is a diagram for illustrating phase matching in the firstpreferred embodiment shown in FIG. 4;

FIG. 6 is a flowchart showing a preferred embodiment of the methodaccording to the present invention;

FIG. 7 is a diagram showing a second preferred embodiment of the NOLMaccording to the present invention;

FIG. 8 is a diagram showing a third preferred embodiment of the NOLMaccording to the present invention;

FIG. 9 is a diagram showing a fourth preferred embodiment of the NOLMaccording to the present invention;

FIG. 10 is a block diagram showing a first preferred embodiment of thesystem according to the present invention;

FIG. 11 is a block diagram showing a second preferred embodiment of thesystem according to the present invention; and

FIG. 12 is a block diagram showing a third preferred embodiment of thesystem according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some preferred embodiments of the present invention will now bedescribed in detail with reference to the attached drawings. Throughoutthe drawings, substantially the same parts are denoted by the samereference numerals.

Referring to FIG. 1, there is shown a configuration of an MZI typeoptical gate in the prior art. This optical gate has a Mach-Zehnderinterferometer including two nonlinear optical media NL-1 and NL-2 eachfor providing a phase shift. Probe light having a wavelength λprobe isinput equally into the nonlinear optical media NL-1 and NL-2. The probelight may be continuous wave (CW) light or optical pulses. In thisexample shown, the probe light is CW light.

An optical signal having a wavelength λsig is input asymmetrically intothe nonlinear optical media NL-1 and NL-2. In this example, the opticalsignal is provided as optical pulses, and it is input into only thenonlinear optical medium NL-1. While the probe light and the opticalsignal are input in the same direction as shown, they may be input inopposite directions.

The nonlinear phase shifts of the probe light in the nonlinear opticalmedia NL-1 and NL-2 while inputting the optical signal are set to becomeφ1 and φ2, respectively. By setting a suitable optical path length ofthe Mach-Zehnder interferometer, the intensity of a converted opticalsignal to be output from this optical gate is proportional to[1−cos(φ1−φ2)]/2. The wavelength of the converted optical signal isequal to the wavelength of the probe light, i.e., equal to λprobe.

Assuming that an optical Kerr effect (cross-phase modulation (XPM) bythe probe light and the optical signal) is used as the nonlinear effect,the phase shift φ is proportional to (γPL)² where γ is the nonlinearcoefficient of each nonlinear optical medium, P is the optical power ineach nonlinear optical medium, and L is the length of interaction of theoptical Kerr effect in each nonlinear optical medium. Thus, the phaseshift φ is proportional to the square of the optical power P, so thatthe input-output characteristic of the optical gate shown in FIG. 1 isas shown in FIG. 2.

In FIG. 2, the vertical axis represents the power Pout of the convertedoptical signal, and the horizontal axis represents the power Psig of theinput optical signal. The horizontal axis is marked with phase shiftscorresponding to input powers. The power Pout of the converted opticalsignal takes on minimum values at 2nπ (n is an integer) for the phaseshift. As a result, the input optical signal can be converted into theconverted optical signal. At this time, the wavelength is converted fromλsig into λprobe (λprobe≠λsig).

In the wavelength conversion using the MZI type optical gate, theconversion is performed in the form proportional to [1−cos(φ1−φ2)]/2.Accordingly, noise in the input optical signal can be partiallysuppressed.

Referring to FIG. 3, there is shown a configuration of an NOLM in theprior art. This NOLM includes a first optical coupler 6 including firstand second optical paths 2 and 4 directionally coupled to each other, aloop optical path 8 for connecting the first and second optical paths 2and 4, and a second optical coupler 12 including a third optical path 10directionally coupled to the loop optical path 8. A part or the whole ofthe loop optical path 8 is provided by a nonlinear optical medium NL.The coupling ratio of the first optical coupler 6 is set to 1:1.

The operation of this NOLM will now be described in brief. When probelight having a wavelength λprobe is input into the first optical path 2of the optical coupler 6 and an optical signal having a wavelength λsigis input into the third optical path 10 of the optical coupler 12, aconverted optical signal having a wavelength λprobe is output from thesecond optical path 4 of the optical coupler 6.

The probe light is divided into two components having the same power bythe optical coupler 6. The two components propagate in the loop opticalpath 8 clockwise and counterclockwise, respectively, and are nextsubjected to a phase shift φ for each by the nonlinear optical mediumNL. Thereafter, they are combined by the optical coupler 6. In combiningthese components at the optical coupler 6, they are equal in power andphase to each other, so that resultant light obtained by thiscombination is output from the first optical path 2 but not output fromthe second optical path 4 as if it is reflected by a mirror.

When an optical signal is input from the middle of the loop optical path8 by the optical coupler 12, this optical signal propagates in the loopoptical path 8 in only one direction thereof (e.g., clockwise in FIG.3), and the nonlinear refractive index of the nonlinear optical mediumNL changes for the light propagating in this direction only whenon-pulses pass therethrough. Accordingly, in combining the twocomponents of the probe light at the optical coupler 6, the phases ofthe two components of the probe light at their portions synchronous withoff-pulses of the optical signal are coincident with each other, and thephases of the two components of the probe light at their portionssynchronous with on-pulses of the optical signal are different from eachother. Letting Δφ denote a phase difference in the latter case, anoutput proportional to [1−cos(Δφ)]/2 is obtained from the second opticalpath 4 of the optical coupler 6.

By setting the power of the input optical signal so that the phasedifference becomes π, it is possible to perform a switching operationsuch that the two components combined upon passing of the on-pulses areoutput only from the second optical path 4. Thus, the conversion fromthe optical signal having the wavelength λsig into the converted opticalsignal having the wavelength λprobe is performed. Like the MZI typeoptical gate shown in FIG. 1, noise can be suppressed also in the NOLMshown in FIG. 3.

FIG. 4 is a diagram showing a first preferred embodiment of the NOLMaccording to the present invention. According to the present invention,the loop optical path 8 is provided by an optical fiber as a nonlinearoptical medium. The optical fiber has a nonlinear coefficient largeenough to reduce the length of the optical fiber to such an extent thatthe optical fiber has a polarization maintaining ability, for example.Particularly in this preferred embodiment, the optical fiber forming theloop optical path 8 is provided by a highly-nonlinear dispersion-shiftedfiber (HNL-DSF). With this configuration, it is possible to realizeoptical signal processing such as ultra high-speed and ultra wide-bandwavelength conversion and optical 2R repeating using a noise suppressingfunction, in which “2R” means two functions of reshaping (waveformequalization) and reamplification (amplitude regeneration).

A switching operation by an optical signal in this preferred embodimentshown in FIG. 4 is similar to that described with reference to FIG. 3,so the description thereof will be omitted herein.

As a nonlinear optical effect applicable to optical signal processing inan optical communication system, it is considered to apply three-wavemixing in a second-order nonlinear optical medium or an optical Kerreffect such as self-phase modulation (SPM), cross-phase modulation(XPM), and four-wave mixing (FWM) in a third-order nonlinear opticalmedium. Examples of the second-order nonlinear optical medium includeInGaAs and LiNbO3. Examples of the third-order nonlinear optical mediuminclude an optical fiber and a semiconductor medium such as asemiconductor optical amplifier (SOA) and a distributed feedback laserdiode (DFB-LD).

The present invention employs an optical Kerr effect in an opticalfiber. A single-mode fiber is suitable as the optical fiber, andespecially a dispersion-shifted fiber (DSF) having a relatively smallchromatic dispersion is preferable.

In general, the third-order nonlinear coefficient γ of an optical fiberis expressed as follows:

γ=ωn ₂ /cA _(eff)  (1)

where ω is the optical angular frequency, c is the velocity of light ina vacuum, and n₂ and A_(eff) are the nonlinear refractive index and theeffective core area of the optical fiber, respectively.

The nonlinear coefficient γ of a conventional DSF is as small as about2.6 W⁻¹km⁻¹, so a fiber length of several km to 10 km or more isnecessary to obtain sufficient conversion efficiency. If a shorter DSFcan be used to realize sufficient conversion efficiency, thezero-dispersion wavelength can be managed with high accuracy, therebyrealizing high-speed and wide-band conversion.

In general, for enhancement of the third-order nonlinear effect of anoptical fiber, it is effective to increase a light intensity byincreasing the nonlinear refractive index n₂ in Eq. (1) or by reducing amode field diameter (MFD) corresponding to the effective core areaA_(eff) in Eq. (1).

The nonlinear refractive index n₂ can be increased by doping the cladwith fluorine or the like or by doping the core with a highconcentration of GeO₂, for example. By doping the core with 25 to 30 mol% of GeO₂, a large value of 5×10⁻²⁰ m²/W or more (about 3.2×10⁻²⁰ m²/Wfor a usual silica fiber) can be obtained as the nonlinear refractiveindex n₂.

On the other hand, the MFD can be reduced by designing a relativerefractive-index difference Δ between the core and the clad or bydesigning the core shape. Such design of a DSF is similar to that of adispersion compensating fiber (DCF). For example, by doping the corewith 25 to 30 mol % of GeO₂ and setting the relative refractive-indexdifference Δ to 2.5 to 3.0%, a small value of less than 4 μm can beobtained as the MFD. Owing to the combined effects of increasing thenonlinear refractive index n₂ and reducing the MFD, an optical fiber(HNL-DSF) having a large value of 15 W⁻¹km⁻¹ or more as the nonlinearcoefficient γ can be obtained.

As another important factor, the HNL-DSF having a large nonlinearcoefficient γ as mentioned above has a zero dispersion in a wavelengthband used. This point can also be satisfied by setting each parameter inthe following manner. That is, in general, a dispersion in a usual DCFincreases in a normal dispersion region with an increase in refractiveindex difference Δ under the condition that the MFD is set constant. Onthe other hand, the dispersion decreases with an increase in corediameter, whereas the dispersion increases with a decrease in corediameter. Accordingly, the dispersion can be reduced to zero byincreasing the core diameter under the condition that the MFD is set toa certain value in a wavelength band used.

A phase shift due to the optical Kerr effect in an optical fiber havinga length L is proportional to γP_(p)L where P_(p) is the average pumplight power. Accordingly, the fiber having a nonlinear coefficient γ of15 W⁻¹km⁻¹ can achieve the same conversion efficiency as that by a usualDSF even when the fiber length is reduced to about 2.6/15≈1/5.7 ascompared with the usual DSF. As mentioned above, the usual DSF requiresa length of about 10 km for sufficient conversion efficiency. To thecontrary, the HNL-DSF having a large nonlinear coefficient γ asmentioned above can obtain a similar effect with a reduced length ofabout 1 to 2 km. Practically, loss in the fiber is reduced in an amountcorresponding to a decrease in fiber length, so that the fiber can befurther shortened to obtain the same efficiency. Thus in a short fiber,controllability of the zero-dispersion wavelength can be improved, andultra wide-band conversion can be achieved as will be hereinafterdescribed. Further, when the fiber length is several km, polarizationcan be fixed, that is, a polarization maintaining ability can beensured. Therefore, application of the HNL-DSF to the present inventionis greatly effective in achieving high conversion efficiency and wideconversion band and removing polarization dependence.

To effectively produce an optical Kerr effect, especially XPM by usingan optical fiber and improve the efficiency of conversion from theoptical signal into the converted optical signal, phase matching betweenthe probe light and the optical signal must be achieved. The phasematching will now be described with reference to FIG. 5.

FIG. 5 is a diagram for illustrating the phase matching in the firstpreferred embodiment shown in FIG. 4. It is now assumed that both theprobe light having a wavelength λprobe supplied to the optical path 2and the optical signal having a wavelength λsig supplied to the opticalpath 10 are optical pulses. The optical pulses as the probe light arebranched to first probe pulses propagating clockwise in the loop opticalpath 8 and second probe pulses propagating counterclockwise in the loopoptical path 8 by the optical coupler 6. The optical pulses as theoptical signal are passed through the optical coupler 12 and propagateclockwise as signal pulses in the loop optical path 8.

A phase matching condition in the loop optical path 8 is given by timingcoincidence of the signal pulses and the first probe pulses bothpropagating clockwise in the loop optical path 8. If the timingcoincidence of the signal pulses and the first probe pulses is notachieved, optical Kerr shift by XPM is limited to cause a difficulty ofeffective switch operation or gate operation.

Since the wavelength of the signal pulses and the wavelength of thefirst probe pulses are different from each other, the group velocity ofthe signal pulses and the group velocity of the first probe pulses aredifferent from each other, resulting in occurrence of timing deviationproportional to the length of the loop optical path 8. To avoid thispossibility, wavelength location is preferably selected so that thegroup velocity of the signal pulses and the first probe pulses becomeequal to each other.

The most effective wavelength location for minimizing the timingdeviation is obtained by locating the wavelength of the signal pulsesand the wavelength of the first probe pulses in substantiallysymmetrical relationship with respect to the zero-dispersion wavelengthof the loop optical path 8. Over a wide band near the zero-dispersionwavelength, the chromatic dispersion changes substantially linearly, sothat a good phase matching condition can be obtained by making the groupvelocities of the signal pulses and the first probe pulses coincide witheach other by the above-mentioned wavelength location.

Thus according to an aspect of the present invention, the phase matchingcondition can be obtained by satisfying the relation of λsig+λprobe=2λ₀where λ₀ is the zero-dispersion wavelength of the loop optical path,thus improving the efficiency of conversion from the optical signal intothe converted optical signal.

However, if there are variations in the zero-dispersion wavelengthitself along the fiber, the group velocities become different from eachother in spite of the above wavelength location, causing a limit to aconversion band and a convertible signal rate. Thus, a conversion bandby the fiber is limited by dispersion. If dispersion along the fiber isperfectly controlled, for example, if a fiber having a zero-dispersionwavelength uniform over the entire length (exactly, the nonlinearlength) is fabricated, a conversion band infinite in fact (unlimitedlywide in a range where the wavelength dependence of dispersion is linear)could be obtained by locating the wavelengths of the probe light and theoptical signal in symmetrical relationship with respect to this uniformzero-dispersion wavelength. Actually, however, the zero-dispersionwavelength varies along the fiber, causing a deviation of the phasematching condition from an ideal condition to result in a limit of theconversion band.

A first method for realizing a wide conversion band is to use anHNL-DSF. In the case that the HNL-DSF is used, sufficient conversion canbe achieved with a length of about 1 to 2 km, so that dispersioncontrollability can be improved to easily obtain a wide-bandcharacteristic. In particular, by suppressing variations in thezero-dispersion wavelength near an input end where the efficiency ofproduction of an optical Kerr effect is high, the conversion band can bewidened most efficiently. Further, by cutting the fiber into a pluralityof small sections and next joining any of the small sections similar inzero-dispersion wavelength by splicing or the like (in an orderdifferent from the initial order counted from a fiber end), a wideconversion band can be obtained although an average dispersion over theentire length is unchanged.

Alternatively, many fibers each having a length (e.g., hundreds ofmeters or less) allowing high-accuracy dispersion control required toobtain a sufficiently wide conversion band may be prepared in advance,and any of these fibers having a required zero-dispersion wavelength maybe combined to be spliced, thereby fabricating a fiber having a lengthrequired to obtain a required conversion efficiency.

In the case of widening the conversion band as mentioned above, it iseffective to gather the sections of the fiber having less variations inzero-dispersion wavelength near an input end (e.g., both ends of anonlinear optical medium) where the light intensity is high. Further,the conversion band can be further widened by increasing the number ofsections of the fiber as required, or by alternately arranging thepositive and negative signs of dispersion at a relativelylarge-dispersion portion separate from the input end to thereby suitablycombine the small sections.

The degree of reducing the length of each section in cutting the opticalfiber may be based on the nonlinear length, for example. The phasematching in FWM in a fiber sufficiently shorter than the nonlinearlength may be considered to depend on the average dispersion of thefiber. As an example, in FWM using a pump light power of about 30 mW ina fiber having a nonlinear coefficient γ of 2.6 W⁻¹km⁻¹, the nonlinearlength is about 12.8 km. In this example, the length of each section isset to about {fraction (1/10)} of 12.8 km, i.e., about 1 km. As anotherexample, in FWM using a pump light power of about 30 mW in a fiberhaving a nonlinear coefficient γ of 15 W⁻¹km⁻¹, the nonlinear length isabout 2.2 km. In this example, the length of each section is set toabout {fraction (1/10)} of 2.2 km, i.e., about 200 m. In any case, awide conversion band can be obtained by measuring an averagezero-dispersion wavelength of fiber sections each sufficiently shorterthan the nonlinear length and combining any of the fiber sections havingalmost the same zero-dispersion wavelength to thereby configure a fiberachieving a required conversion efficiency.

Thus according to the present invention, there is provided a firstmethod for manufacturing an NOLM having a nonlinear optical medium toobtain the function of an optical gate. In this method, an optical fiberis first cut into a plurality of sections, and the plural sections arenext arranged to be joined together so that a conversion band by athird-order nonlinear effect using a nonlinear optical medium becomesmaximum, thereby obtaining the nonlinear optical medium. This nonlinearoptical medium is used to configure an NOLM, and probe light is used toconvert an optical signal into a converted optical signal, therebyobtaining a wide conversion band.

Preferably, dispersions of the plural sections are measured, and theplural sections are arranged so that any of these sections having lessvariations in zero-dispersion wavelength are located near the input andoutput ends of the nonlinear optical medium. With this configuration, aphase matching condition can be effectively obtained at a fiber portionwhere optical power is high, so that the conversion band can beeffectively widened.

Preferably, at least a part of the plural sections is joined so that thepositive and negative signs of dispersion are alternately arranged. Withthis configuration, the average dispersion of each part of the opticalfiber can be suppressed to thereby effectively widen the conversionband.

Further, according to the present invention, there is provided a secondmethod for manufacturing an NOLM having a nonlinear optical medium toobtain the function of an optical gate. In this method, an optical fiberis first cut into a plurality of sections, and a dispersion of each ofthe plural sections is next measured. Thereafter, any of the pluralsections having dispersions small enough to obtain a required conversionband by a third-order nonlinear effect using a nonlinear optical mediumare selected and joined together to thereby obtain the nonlinear opticalmedium. This nonlinear optical medium is used to configure an NOLM, andprobe light is used to convert an optical signal into a convertedoptical signal, thereby obtaining a wide conversion band.

While the optical fiber is first cut into a plurality of sections ineach of the first and second methods according to the present invention,the present invention is not limited to this technique. For example, theoptical fiber may be cut as required in the following manner.

According to the present invention, there is provided a third method formanufacturing an NOLM having a nonlinear optical medium to obtain thefunction of an optical gate. In this method, a deviation inzero-dispersion wavelength of an optical fiber is measured, and in thecase that the measured deviation exceeds a predetermined range, theoptical fiber is cut into a plurality of sections to make a deviation inzero-dispersion wavelength of each section fall within the predeterminedrange. Thereafter, the optical fiber or the sections havingsubstantially the same zero-dispersion wavelength is/are selected to bejoined together, thereby obtaining a nonlinear optical medium. Thisnonlinear optical medium is used to configure an NOLM, and probe lightis used to convert an optical signal into a converted optical signal,thus obtaining a wide conversion band.

The measurement of the zero-dispersion wavelength may be performed byusing the fact that the efficiency of generation of FWM differsaccording to the zero-dispersion wavelength, for example. In general,chromatic dispersion can be obtained by measuring the wavelengthdependence of group velocity, and the best phase matching condition inFWM is obtained when a pump light wavelength and a zero-dispersionwavelength coincide with each other. Accordingly, the zero-dispersionwavelength can be obtained as a pump light wavelength giving a maximumefficiency of generation of FWM by measuring the efficiency ofgeneration of FWM to the pump light wavelength in the condition that awavelength difference between pump light and signal light is set to arelatively large constant value of about 10 to 20 nm, for example.

Further, the efficiency of generation of FWM is proportional to thesquare of pump light intensity. Accordingly, in the case that thezero-dispersion wavelength varies along an optical fiber, thezero-dispersion wavelength measured in the case that signal light andpump light are input into the optical fiber from its one end isgenerally different from that measured in the case that they are inputinto the optical fiber from its other end. Accordingly, a deviation inzero-dispersion wavelength of the optical fiber can be obtainedaccording to these two different measured values of the zero-dispersionwavelength. This will now be described more specifically.

Referring to FIG. 6, there is shown a process 14 for manufacturing anonlinear optical medium having a small deviation in zero-dispersionwavelength. In step 16, a tolerance Δλ₀ of zero-dispersion wavelength isdetermined. The tolerance Δλ₀ may be determined as a characteristicrequired by a system from a required conversion band, and its specificvalue is 2 nm, for example.

In step 18, a deviation δλ in zero-dispersion wavelength is measured.For example, when an optical fiber F1 is given, a zero-dispersionwavelength λ₀₁ obtained in the case of inputting signal light and pumplight into the optical fiber Fl from its first end and a zero-dispersionwavelength λ₀₂ obtained in the case of inputting signal light and pumplight into the optical fiber F1 from its second end are measured fromthe efficiency of generation of FWM as mentioned above. In this case,the deviation δλ in zero-dispersion wavelength can be replaced by avalue of |λ₀₁ 31 λ₀₂|.

In step 20, it is determined whether or not the deviation δλ is smallerthan the tolerance Δλ₀. Assuming that the relation of Δλ₀≦δλ issatisfied in the optical fiber F1, the optical fiber F1 is cut into twooptical fibers F1A and F1B in step 22.

After step 22, the flow returns to step 18, in which a deviation δλ ineach of the optical fibers F1A and F1B is measured. Thereafter, thedecision is made for each measured value in step 20. Assuming that eachmeasured value of the deviation δλ is smaller than Δλ₀ in the opticalfibers F1A and F1B, this flow is ended. A cutting position of theoptical fiber F1 in step 22 is arbitrary. Accordingly, the lengths ofthe optical fibers F1A and F1B may be equal to each other or differentfrom each other.

While steps 18 and 20 are once repeated in the above description, steps18 and 20 may not be repeated or may be repeated more times. Forexample, in the case that an optical fiber F2 having a small deviationin zero-dispersion wavelength is given, the condition is satisfied instep 20 of the first cycle. In this case, the optical fiber F2 is notcut. Conversely, in the case that an optical fiber F3 having largevariations in zero-dispersion wavelength along the fiber is given, theoptical fiber F3 is cut into two optical fibers F3A and F3B in step 22of the first cycle. In the case that the optical fiber F3A satisfies thecondition in step 20 of the second cycle, but the optical fiber F3B doesnot satisfy the condition in step 20 of the second cycle, the opticalfiber F3B is cut into two optical fibers F3B1 and F3B2 in step 22 of thesecond cycle. Then, the process 14 is ended. In this case, the threeoptical fibers F3A, F3B1, and F3B2 are obtained from the originaloptical fiber F3, so that the deviation in zero-dispersion wavelength ofeach fiber is smaller than the tolerance Δλ₀.

The plural optical fiber sections (the optical fibers F1A, F1B, and soon) thus obtained are arranged according to the measured values of thezero-dispersion wavelength, and any of the optical fibers havingsubstantially the same zero-dispersion wavelength are selected andjointed together to obtain a length giving a required conversionefficiency, thus obtaining a nonlinear optical medium having greatlyreduced variations in zero-dispersion wavelength along the fiber. Thisnonlinear optical medium is used to configure an NOLM, thus obtaining awide conversion band.

Although the zero-dispersion wavelengths λ₀₁ and λ₀₂ are substantiallyequal to each other, the optical fiber may have large variations inzero-dispersion wavelength along the fiber. For example, there is a casethat the distribution of the zero-dispersion wavelength along the fiberis symmetrical with respect to the longitudinal center of the fiber. Inthis case, the optical fiber is cut into at least two sections prior tostarting of the process 14. Then, the process 14 is carried out for eachsection. Alternatively, the process 14 may be repeated plural times.

As mentioned above, it is effective to set the zero-dispersionwavelength of a fiber and the wavelength of pump light substantiallyequal to each other for the generation of FWM. However, if the power ofpump light, signal light, or converted light exceeds a threshold valueof stimulated Brillouin scattering (SBS) in the fiber, the efficiency ofgeneration of FWM is reduced. To suppress the effect of SBS, the pumplight or signal light is subjected to frequency modulation or phasemodulation. In such modulation, a modulating rate of hundreds of kHz isgood enough, and in the case that the signal light is a high-speedsignal having a signal rate on the order of Gb/s, the modulation hasalmost no adverse effect on the signal light.

In the preferred embodiment shown in FIG. 4, the loop optical path 8 isconfigured from an HNL-DSF. In the HNL-DSF, its third-order nonlinearcoefficient can be increased 5 to 10 times that of a conventional DSF,so that the product of the length and an optical power required to setthe phase difference Δφ to π can be reduced to ⅕ to {fraction (1/10)}.Accordingly, a required length for the same signal power can be reducedto ⅕ to {fraction (1/10)}, with the result that a sufficientcharacteristic can be obtained with a reduced length of 1 km or less. Asa result, it is possible to provide an NOLM which can suppress a signalrate limit due to chromatic dispersion, can eliminate the polarizationdependence of an input optical signal, and can eliminate the need formeasures against polarization fluctuations in the loop optical path 8.

FIG. 7 is a diagram showing a second preferred embodiment of the NOLMaccording to the present invention. In this preferred embodiment, theloop optical path 8 is composed of half portions 8-1 and 8-2 each givinga phase shift Δφ/2. Each of the half portions 8-1 and 8-2 is configuredfrom an HNL-DSF of polarization maintaining fiber (PMF) type. The totalphase shift given by both of the half portions 8-1 and 8-2 becomes Δφ,thereby obtaining the function of an optical gate as in the preferredembodiment shown in FIG. 4.

Particularly in this preferred embodiment, a λ/2 plate function 24 fororthogonally intersecting polarization states is added at the midpointof the loop optical path 8, i.e., at a connection point between the halfportions 8-1 and 8-2. The λ/2 plate function 24 can be obtained bysplicing the half portions 8-1 and 8-2 in such a manner that theprincipal axes of the half portions 8-1 and 8-2 orthogonally intersecteach other. With this configuration, the conversion efficiency does notdepend on the polarization state of an input optical signal.Furthermore, the addition of the λ/2 plate function 24 allowssuppression of polarization mode dispersion due to different groupvelocities of two polarization modes of each polarization maintainingfiber. More specifically, by 45° inclining the polarization plane ofprobe light to be introduced through the optical coupler 6 into the loopoptical path 8 with respect to the principal axis of each polarizationmaintaining fiber, it is possible to obtain a conversion efficiency notdepending upon the polarization state of an optical signal to beintroduced through the optical coupler 12 into the loop optical path 8.

The conversion efficiency is defined as a ratio between the power of aninput optical signal to be introduced through the optical coupler 12into the loop optical path 8 and the power of a converted optical signalto be taken from the loop optical path 8 through the optical coupler 6.

FIG. 8 is a diagram showing a third preferred embodiment of the NOLMaccording to the present invention. Since the operation of an opticalgate in the NOLM depends on the magnitude of a phase shift in an opticalKerr effect, especially, XPM, it is preferable to make adjustable thepowers of an input optical signal and probe light to be introduced intothe loop optical path 8. In this respect, this preferred embodimentemploys a power controller 26 for adjusting the power of the probe lightand a power controller 28 for adjusting the power of the input opticalsignal. A variable optical attenuator or an optical amplifier having avariable gain may be used as each of the power controllers 26 and 28.

Further, optical filters 30, 32, and 34 are used to suppress noise lightoutside the band of the probe light, signal light, or converted light.The optical filter 30 is provided between the power controller 26 andthe first optical path 2 of the optical coupler 6 to act on the probelight to be introduced through the optical coupler 6 into the loopoptical path 8. An optical bandpass filter having a pass band includingthe wavelength λprobe of the probe light may be used as the opticalfilter 30.

The optical filter 32 is provided between the power controller 28 andthe third optical path 10 of the optical coupler 12 to act on the inputoptical signal to be introduced through the optical coupler 12 into theloop optical path 8. An optical bandpass filter having a pass bandincluding the wavelength λsig of the input optical signal or an opticalband-rejection filter having a rejection band including the wavelengthλprobe of the probe light may be used as the optical filter 32. Thereason why the SNR of the converted optical signal is improved also inthe case of using such an optical band-rejection filter is such thatsince an optical signal to be subjected to optical gate processinggenerally accompanies ASE noise due to transmission, the SNR can beimproved by preliminarily removing the ASE noise near the wavelengthλprobe of the converted optical signal.

The optical filter 34 is connected to the second optical path 4 of theoptical coupler 6 to act on the converted optical signal output from theloop optical path 8 through the optical coupler 6. The optical filter 34may be provided by an optical bandpass filter having a pass bandincluding the wavelength λprobe of the converted optical signal or by anoptical band-rejection filter having a rejection band including thewavelength λsig of the input optical signal. The center wavelength inthe pass band or the rejection band of each filter is coincident withthe center wavelength of the probe light or the center wavelength of theinput optical signal. The width of the pass band or the rejection bandof each filter is substantially equal to or slightly greater than thatof the band of the input optical signal. Specific examples of eachfilter include a dielectric multilayer filter and a fiber gratingfilter.

FIG. 9 is a diagram showing a fourth preferred embodiment of the NOLMaccording to the present invention. In this preferred embodiment, apower controller 26 for adjusting the power of probe light and a powercontroller 28 for adjusting the power of an input optical signal areautomatically controlled by a control circuit 36. For example, thecontrol circuit 36 controls at least one of the power controllers 26 and28 according to an output signal from a power monitor 40 for receiving apart of a converted optical signal which part is extracted from thesecond optical path 4 of the optical coupler 6 by an optical coupler 38so that the power of the converted optical signal detected by the powermonitor 40 is increased.

Alternatively, the control circuit 36 may control at least one of thepower controllers 26 and 28 according to an output signal from a powermonitor 44 for receiving a part of light output from the first opticalpath 2 of the optical coupler 6 in a direction opposite to thepropagation direction of the probe light by an optical coupler 42 sothat the power detected by the power monitor 44 is reduced.

With this configuration, the power of at least one of the input opticalsignal and the probe light can be controlled so that a proper phasedifference is produced in the loop optical path 8. Accordingly, a highconversion efficiency can be automatically maintained.

FIG. 10 is a block diagram showing a first preferred embodiment of thesystem according to the present invention. This system has an opticalgate device 46. The optical gate device 46 may be provided by any one ofthe above preferred embodiments of the NOLM according to the presentinvention. The optical gate device 46 has an input port 46A for probelight, an output port 46B for a converted optical signal, and an inputport 46C for an input optical signal. The ports 46A, 46B, and 46Ccorrespond to the first, second, and third optical paths 2, 4, and 10shown in FIG. 4, respectively, for example. A probe light source 48 isconnected to the port 46A, and probe light Eprobe output from the probelight source 48 is supplied to the optical gate device 46. A firstoptical fiber transmission line 50 is connected to the port 46C, and anoptical signal Es transmitted by the optical fiber transmission line 50is supplied to the optical gate device 46. A second optical fibertransmission line 52 is connected to the port 46B, and a convertedoptical signal Ec output from the optical gate device 46 is transmittedby the second optical fiber transmission line 52.

Particularly in this preferred embodiment, a transmitting station 54 isprovided to supply the optical signal Es to the first optical fibertransmission line 50, and a receiving station 56 is provided to receivethe converted optical signal Ec transmitted by the second optical fibertransmission line 52.

As a modulating method for an optical signal in the transmitting station54, optical amplitude (intensity) modulation is adopted, for example. Inthis case, direct detection may be adopted as demodulation in thereceiving station 56, for example.

Each of the optical fiber transmission lines 50 and 52 may be providedby a single-mode fiber, 1.3-μm zero-dispersion fiber, or 1.55-μmdispersion-shifted fiber.

By configurating an HNL-DSF used as a nonlinear optical medium in theoptical gate device 46 into a single-mode type and setting the modefield diameter of the HNL-DSF smaller than the mode field diameter ofeach of the optical fiber transmission lines 50 and 52, it is possibleto obtain a nonlinear coefficient large enough to reduce the length ofthe HNL-DSF.

According to this system, a gate operation based on the optical signalEs and the probe light Eprobe can be performed in the optical gatedevice 46, and the converted optical signal Ec obtained by wavelengthconversion applied to the optical signal Es from the first optical fibertransmission line 50 according to the gate operation can be transmittedby the second optical fiber transmission line 52.

Although not shown, one or more optical amplifiers may be arranged alongan optical path including the optical fiber transmission lines 50 and52. In the case that an erbium doped fiber amplifier (EDFA) is used aseach optical amplifier, ASE noise is generated in each optical amplifierand accumulated along the optical path. According to the system shown inFIG. 10, however, the SNR can be improved in accordance with theabove-mentioned principle of noise suppression in the optical gatedevice 46.

While the optical gate device 46 is used as a repeater in this preferredembodiment, an optical gate device may be provided in the receivingstation 56, thereby improving eye opening of the detected signal.

Although not shown, the system shown in FIG. 10 may further has adispersion compensator for compensating for dispersion of at least oneof the optical fiber transmission lines 50 and 52. The dispersioncompensator provides dispersion having a sign opposite to the sign ofthe dispersion of each optical fiber transmission line, for example. Theabsolute value of the dispersion provided by the dispersion compensatoris adjusted so that a receiving condition in the receiving station 56becomes optimum. The use of the dispersion compensator allowssuppression of chromatic dispersion occurring in each optical fibertransmission line, thereby achieving long-haul transmission.

FIG. 11 is a block diagram showing a second preferred embodiment of thesystem according to the present invention. In this preferred embodiment,an input end of a first optical fiber transmission line 50 is connectedto an optical multiplexer 58. Four channels of optical signals Es1, Es2,Es3, and Es4 respectively transmitted from four optical transmitters 60(#1, #2, #3, and #4) are supplied respectively through four opticaldelay circuits 62 (#1, #2, #3, and #4) to the optical multiplexer 58. Inthe optical delay circuits (#1 to #4), the optical signals Es1 to Es4are adjusted in position along a time axis.

The optical signals Es1, Es2, Es3, and Es4 have wavelengths λs1, λs2,λs3, and λs4, respectively, which are different from each other. Theoptical signals Es1 to Es4 are obtained by intensity modulation by shortpulses each having a pulse duration sufficiently shorter than a datarepetition time T. These optical signals Es1 to Es4 are shiftedsequentially by T/4 by the optical delay circuits 62 (#1 to #4).Accordingly, a wavelength division multiplexed signal including theoptical signals Es1 to Es4 shifted along the time axis is output fromthe optical multiplexer 58.

When the wavelength division multiplexed signal is supplied to theoptical gate device 46, all the wavelengths of the four channels areconverted into the wavelength λprobe of the probe light. Accordingly,the converted optical signal to be output from the optical gate device46 to the optical fiber transmission line 52 becomes a time divisionmultiplexed signal.

Thus, according to the system shown in FIG. 11, a wavelength divisionmultiplexed signal can be converted into a time division multiplexedsignal.

While the four-channel wavelength division multiplexed signal is used inthis preferred embodiment, the number of channels is not limited tofour. For example, in the case that an N-channel (N is an integergreater than 1) wavelength division multiplexed signal is used, anN-channel time division multiplexed signal can be obtained. In thiscase, N optical delay circuits are used, and the time shift therebetweenis set to T/N.

FIG. 12 is a block diagram showing a third preferred embodiment of thesystem according to the present invention. In this preferred embodiment,the time division multiplexed signal obtained in the system shown inFIG. 11 is supplied to an optical gate device 46 by a first opticalfiber transmission line 50, and probe light given by optical pulsessynchronized with any channel of the time division multiplexed signal issupplied from a probe light source 48 to the optical gate device 46.With this configuration, an optical signal of only the channelsynchronous with the probe light is converted into a converted opticalsignal, so that a demultiplexing or add/drop operation for the timedivision multiplexed signal can be performed.

According to the present invention as described above, it is possible toprovide an optical gate device allowing the use of a relatively shortoptical fiber as a nonlinear optical medium, and also to provide amanufacturing method for the device and a system including the device.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. A device comprising: a first optical couplerincluding first and second optical paths directionally coupled to eachother; a loop optical path for connecting said first and second opticalpaths, said loop optical path including an optical fiber as a nonlinearoptical medium; a second optical coupler including a third optical pathdirectionally coupled to said loop optical path, said optical fiberhaving a nonlinear coefficient large enough to reduce the length of saidoptical fiber to such an extent that said optical fiber has apolarization maintaining ability; probe light having a first wavelengthis supplied to said first optical path; an optical signal having asecond wavelength different from said first wavelength is supplied tosaid third optical path; a converted optical signal having said firstwavelength and synchronous with said optical signal is output from saidsecond optical path; an optical bandpass filter connected to said thirdoptical path and having a pass band including said second wavelength;and an optical band-rejection filter connected to said third opticalpath and having a rejection band including said first wavelength.
 2. Adevice according to claim 1, wherein said optical fiber includes a coredoped with GeO2 and a clad doped with fluorine.
 3. A device according toclaim 1, wherein: said optical fiber comprises a single-mode fiber; saidsingle-mode fiber having a mode field diameter smaller than the modefield diameter of a single-mode fiber used as a transmission line.
 4. Adevice according to claim 1, further comprising an optical bandpassfilter connected to said second optical path and having a pass bandincluding said first wavelength.
 5. A device according to claim 1,further comprising an optical band-rejection filter connected to saidsecond optical path and having a rejection band including said secondwavelength.
 6. A device according to claim 1, further comprising:detecting means connected to said second optical path for detecting thepower of said converted optical signal; and means for controlling thepower of at least one of said optical signal and said probe light sothat the power detected by said detecting means is increased.
 7. Adevice according to claim 1, further comprising: detecting meansconnected to said first optical path for detecting the power of lighthaving said first wavelength output from said first optical path in adirection opposite to the propagation direction of said probe light; andmeans for controlling the power of at least one of said optical signaland said probe light so that the power detected by said detecting meansis reduced.
 8. A device according to claim 1, wherein: said opticalfiber comprises first and second polarization maintaining fibers; saidfirst and second polarization maintaining fibers being spliced to eachother so that the principal axes of said first and second polarizationmaintaining fibers orthogonally intersect each other.
 9. A manufacturingmethod for a device having a first optical coupler including first andsecond optical paths directionally coupled to each other, a nonlinearoptical medium for forming a loop optical path connecting said first andsecond optical paths, and a second optical coupler including a thirdoptical path directionally coupled to said loop optical path, saidmethod comprising: cutting an optical fiber into a plurality ofsections; and arranging said plurality of sections and joining themtogether so that a conversion band by a third-order nonlinear effectusing said nonlinear optical medium becomes a maximum, thereby obtainingsaid nonlinear optical medium, wherein said arranging includes measuringthe dispersions of said plurality of sections and said plurality ofsections are arranged so that any of said sections having lessvariations in zero dispersion wavelength are located near both ends ofsaid nonlinear optical medium.
 10. A method according to claim 9,wherein at least a part of said plurality of sections is joined so thatthe positive and negative signs of the dispersions of said at least apart become alternate.
 11. A manufacturing method for a device having afirst optical coupler including first and second optical pathsdirectionally coupled to each other, a nonlinear optical medium forforming a loop optical path connecting said first and second opticalpaths, and a second optical coupler including a third optical pathdirectionally coupled to said loop optical path, said method comprising:measuring a deviation in zero-dispersion wavelength of an optical fiber;cutting said optical fiber into a plurality of sections when saiddeviation exceeds a predetermined range, and making the deviation inzero-dispersion wavelength of each of said sections fall within saidpredetermined range; and selecting said optical fiber or said sectionshaving substantially the same zero-dispersion wavelength and joiningsaid selected sections, thereby obtaining said nonlinear optical medium.12. A device made by the process comprising: cutting an optical fiberinto a plurality of sections; and arranging said plurality of sectionsand joining them together so that a conversion band by a third-ordernonlinear effect using a nonlinear optical medium becomes a maximum,thereby obtaining said nonlinear optical medium, wherein said arrangingincludes measuring dispersions of said plurality of sections, and saidplurality of sections are arranged so that any of said sections havingless variations in zero dispersion wavelength are located near both endsof said nonlinear optical medium.
 13. A device made by the processcomprising: measuring a deviation in zero-dispersion wavelength of anoptical fiber; cutting said optical fiber into a plurality of sectionswhen said deviation exceeds a predetermined range, and making thedeviation in zero-dispersion wavelength of each of said sections fallwithin said predetermined range; and selecting said optical fiber orsaid sections having substantially the same zero-dispersion wavelengthand joining said selected sections, thereby obtaining a nonlinearoptical medium.